The 82-nucleotide identifier (ID) sequence is present in the rat genome in 1-1.5 X 10(5) copies and in cDNA clones of precursors of brain-specific mRNAs. One brain-specific gene contains more than one ID sequence in its introns. There is an excess of ID sequences to brain genes, and some ID sequences appear to have been inserted as mobile elements into other genetic locations. Therefore, brain genes contain ID sequences in their introns, but not all ID sequences are located in brain gene introns. A brain ID consensus sequence has been obtained by comparing 8 ID nucleotide sequences.